A system and method for conditionally interfering with the operation of a conventional toilet tank fill valve prevents toilet overflow and conserves water. A single self-contained apparatus, located completely within the toilet tank, detects and prevents a toilet overflow and/or conserves water by conditionally interfering with or overriding the normal operation of a toilet tank fill valve to prevent a further flush. Exemplary illustrative non-limiting steps include removing the toilet tank lid to provide access to said toilet tank fill valve within the tank, manually snapping an assembly onto said fill valve protective cap, and automatically and conditionally actuating said assembly to cause the assembly to selectively interfere with or override the operation of said toilet tank fill valve. An exemplary illustrative non-limiting device comprises an attaching structure for manual attachment to said fill valve cap, and a control mechanism supported by said attaching structure. The control mechanism acts to conditionally interfere with the operation of the toilet tank fill valve assembly in response to abnormal detected toilet tank water level over time, thereby preventing a further flush. The overflow and water conservation feature can be designed into a fill valve at time of manufacture and/or added in the field by an end user.